1. Field of the Invention
The subject matter of this invention is related generally to motor controllers and is related more specifically to microprocessor-controlled motor controllers.
2. Description of the Prior Art
Motor controllers or motor starters as they are sometimes called are well known in the art. Generally, they comprises a switch or ganged switches which are operable to open or close to provide or interrupt current to the stator windings, for example, of an electric motor. The opening and closing process is controlled by appropriately arranged relay coils and relay contacts in which is well known in the prior art as a "relay ladder" arranged in a logical order to properly sequence the starting and/or stopping of the motor.
With the advent of the microprocessor it was found that part, if not all of the relay logic arrangement, could be replaced by a properly programmed microprocessor. Such arrangements are shown and described in a paper entitled "A Quantative Analysis of Grouped Single-Phase Induction Motors" published on page 125 of the IEEE Transactions on Industry Applications, Vol. 1A-17 No. 2, March/April 1981 by J. R. Dunki-Jacobs and Robert H. Kerr; a paper entitled "Thermal Tracking-A Rational Approach to Motor Protection" by D. R. Boothman, E. C. Elgar, R. H. Rehder and R. J. Wooddall identified as IEEE Transactions Paper 274029-5 recommended for presentation at the IEEE PES Winter Meeting, New York, N.Y., Jan. 27-Feb. 1, 1974; a paper entitled "Microprocessor-Based Universal Motor Protection System" appearing in the IEEE Transactions on Industry Applications, Vol. 1A-17 No. 1, January/February 1981 by E. B. Turner and H. Michael Wiley and in a descriptive bulletin (41-560(E)) entitled MOTOGARD.TM. for motor protection by the Westinghouse Canada Switchgear and Control Division issued January 1980. A reading of all of the above will show that certain factors are of importance when discussing motor starters or motor controllers.
In the protection of induction motors, especially large ones, it is important to consider the affect of energy input to the rotor as well as to the stator thereof. That the monitoring of a rotating rotor is difficult does not make an attempt at predicting rotor temperature any less important. A basic thermal equivalent model of a rotor current stator system is well known. With this model the temperature rise of the rotor from a steady-state material condition is exponential in character. The final temperature rise is dependent primarily on the value of the heating current. Furthermore, if there is a limiting temperature which the rotor is not permitted to exceed, this limit must be reached in some predetermined time. In practice it has been found that a key failure mechanism of the rotors of induction motors, especially large types, is differential thermal expansion between the rotor bars and the rotor end bells. This differential thermal expansion results in failure if the temperature differential approaches approximately 200.degree. C. or so. This particularly has an effect on the brazing between the rotor bar and the end bells. It has typically been found that it takes approximately one minute for the aforementioned temperature difference to equalize after a heat source of one kind or another has been removed from the region of the rotor. It would be advantageous if means could be found to utilize the known model in conjunction with approximations related to temperature rise for shutting down or turning off an induction motor if the critical temperature differential is exceeded during the start-up phase of operation. Further it would be advantageous if this could be done using a minimum of existing hardware equipment. It would be further advantageous if the means for utilizing this model of motor protection could be adjustable in a simple way to accommodate different parameters including, for example, manufacturer-supplied stall time information and locked rotor current information and it would be further advantageous if the system discovered could easily interphase with existing temperature measurement devices to provide automatic tracking of conditions as the motor operates after start up.